94 REACTION: THE INFLUE^'CE OF COMMUNITY ON HABITAT 



mittent by contrast with those of the climax. Moreover, his control 

 is conscious and usually intentional, and thus may be extended in 

 space, time, or degree at the behest of need or interest. This is as 

 true of air as of soil reactions, and man has modified or evaded local 

 climates by virtue of shelter, heating and cooling, as well as by a 

 number of minor devices. His effect upon climate in the large has 

 mostly been unintentional or unintelligent, or has dealt with compen- 

 sation, as by the long-distance transport of water for irrigation or 

 urban needs, or the drainage of large areas. However, he is on the 

 threshold of a new era in which the mastery of fallen rain will become 

 more or less complete and even lead to the increasing control of actual 

 rainfall by virtue of great coactions within the plant matrix of the 

 various biomes. This will be brought about by the expanding knowl- 

 edge of climaxes and succession, by which the latter may be widely 

 employed as a unique tool for regulating runoff and erosion. 



The theme of man as a superdorainant and superinfluent is far too 

 vast even to be outlined in its major features as a part of the present 

 treatment, but some of the chief reactions involved are briefly touched 

 upon in the following chapter. (Cf. Sears, 1937.) 



REACTION IN WATER 



Obviously, the change from air and soil on land to water and soil 

 leads to divergence in the manner of reaction, even though the gen- 

 eral processes of adding, subtracting, or modifying are essentially 

 similar. For the most part, the soil becomes merely a substratum for 

 attachment and for the accumulation of detritus, with the loss of its 

 properties as a storehouse of nutrients and gases. At the same time, 

 the air is replaced by a denser and less elastic medium, which must 

 combine in large measure the functions of both media on land. This 

 necessitates a great reduction in the amount of all raw materials in 

 solution, both gaseous and mineral, as compared with soil, for which 

 the slower movement of water provides but slight compensation. Con- 

 sequently, whereas reaction on land centers about water content, in 

 water it is primarily concerned with the amount and distribution of 

 solutes, and suspended matter, and with circulation. 



Reactions in Fresh Water 



The transition from land to water reactions is particularly gradual 

 in ponds and lakes, though a similar gradation occurs in estuaries and 

 false bays. The first three stages of the hydrosere are characterized 

 by three types of vegetation, viz., (a) submerged, (5) floating, and 



